Optical discs such as compact discs (CDs), video compact discs (VCDs) and digital versatile discs (DVDs) are generally produced by recording data on one side thereof with an electroplating process and printing disc- and/or manufacturer-related information on the other side thereof with a coating process. In addition to the difference in recorded data and coated information, the electroplating and coating processes may also vary with the implements of the disc manufacturers, so the painting distribution may differ disc by disc. Currently, even the shape of the disc is variable. An optical disc reading apparatus is required to be able to read data from discs produced by different manufacturers. For example, if the painting distribution of the optical disc is not uniform, vibration is likely rendered when the optical disc is rotating in the disc reading apparatus. Under this circumstance, the optical disc is considered unbalanced. For reading an unbalanced disc, the rotating speed has to be lowered in order to avoid errors.
An unbalanced disc can be detected out by an open-loop test procedure of the optical disc drive. In the open-loop test procedure, the optical pickup head of the optical disc drive is kept unmoved while the optical disc being read. The optical disc is rotated with a rotating shaft, and the number of tracks that the optical pickup head crosses during one single revolution of the disc is computed. In a perfectly ideal situation, there should be only one track of crossing-over detected for each revolution of the disc if the optical pickup head remains fixed. In practice, however, more or less deviation would occur, particular due to vibration of the disc. Under this circumstance, the fixed optical pickup head might relatively cross some tracks of the rotating optical disc. If the number of crossing tracks is greater than a threshold value, it means the vibration is severe and the disc is deemed an unbalanced disc.
The determination of the threshold value will be described hereinafter with reference to FIGS. 1(a) and 1(b), which are schematic perspective and side views illustrating rotation of an unbalanced disc. For purpose of neat drawing, only the rotating shaft 20 and optical pickup head 21 of the optical disc drive are schematically shown interacting with the unbalanced disc 10. For determining the threshold number of crossing tracks for discriminating a normal disc or an unbalanced disc, various unbalanced test discs are respectively subjected to the open-loop test procedure as the disc 10 first. For example, three kinds of test discs having different unbalanced levels and labeled with “3g”, “5g” and “6g” are used, wherein the disc “6g” vibrates more significantly than the disc “5g” and the disc “5g” vibrates more significantly than the disc “3g”. The crossing track numbers of these test discs realized according to a track error (TE) signal under high-speed rotation and certain configurations of discs have been known and shown in the scheme table of FIG. 2. In details, what is represented by the label LSB3g indicates the crossing track number detected in the open-loop test procedure with the “3g” disc standing upright and detected leftward by the optical pickup head; what is represented by the label RSB3g indicates the crossing track number detected in the open-loop test procedure with the “3g” disc standing upright and detected rightward by the optical pickup head; and what is represented by the label HB3g indicates the crossing track number detected in the open-loop test procedure with the “3g” disc placed horizontally above the optical pickup head. These arrangements are used to cover all the possible placement of the optical disc drive. Similar labels LSB5g, HB 5g, RSB5g, LSB6g, HB 6g and RSB6g in FIG. 2 denote similar meanings to those mentioned above.
By rotating the test discs under a predetermined high testing speed of rotation and detecting the resulting tracking error signals with the optical pickup head, respectively, a proper test disc is selected to set up the threshold value for discriminating a normal disc or an unbalanced disc. In the example of FIG. 2, the test disc “3g” operates under the high testing speed of rotation in this optical disc drive no matter what type of placement of the disc drive, but not for all circumstances the discs “5g” and “6g” do. Accordingly, the maximum crossing track number of the disc “3g”, i.e. 225, is used as the threshold value L1 for discriminating a normal disc or an unbalanced disc. In other words, all discs detected with more than 225 crossing tracks per cycle will be considered unbalanced and read under a low reading speed of rotation. On the other hand, all discs detected with less than 225 crossing tracks per revolution will be considered normal and read under the high reading speed of rotation. However, some of the unbalanced discs would not be screened out since range considered normal (crossing track number less than 225) would cover some severe unbalanced disc with other types of placement, e.g. HB5g or HB6g, which is not suitable to operate under the high reading speed of rotation. Therefore, errors still possibly occur.